Apparatus for constructing ring network

ABSTRACT

A first communication apparatus to be coupled to a second communication apparatus and a third communication apparatus via a ring network, including: a first port to coupled to the ring network; and a second port to coupled to the ring network; wherein the first port and the second port is set to receive a control frame from the second communication apparatus and the third communication apparatus, and either the first port or the second port is set to be disabled to relay a user frame.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 11/833,320, filedAug. 3, 2007. This application relates to and claims priority fromJapanese Patent Application No. 2006-321021, filed on Nov. 29, 2006. Theentirety of the contents and subject matter of all of the above isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an apparatus which providesredundancy of communication routes in a ring network. More specifically,the present invention is directed to such an apparatus capable ofrealizing high-speed switching of routes, and capable of simply couplinga plurality of ring networks, and also capable of independentlyperforming operations in each of these ring networks so as to realizeconstructions of more flexible networks, and further, capable ofreducing loads given to a CPU.

2. Description of the Related Art

While networks are constructed, there are some possibilities that aplurality of apparatuses are connected to each other in ring structuresin order to make communication routes redundant. Networks having ringstructures have a feature that necessary amounts of transfer routes andthe like may be reduced, as compared with those of networks having meshstructures. On the other hand, since the networks are structured in ringshapes, when broadcast packets flow through the networks having the ringstructures, transfer operations of these broadcast packets arerepeatedly carried out in an infinite time, so that such a loop isproduced in which the broadcast packets are continuously circulated inthe same route for a long time. As a result, traffic loads on thenetworks caused by broadcast streams are increased, or CPU loads causedby unstable conditions of learning are increased, which may give adverseinfluences to communications. In order to avoid the occurrence of thisloop, various methods capable of logically cutting off loop structureshave been proposed in networks having ring structures.

In the spanning tree protocol described in IEEE Std 802.1D, 1998Edition, such a control frame called as “BPDU” is periodically exchangedamong apparatuses which construct a network in order to cut off a loopstructure. While a plurality of protocol information used to determineoperations of the protocol have been superimposed on the BPDU, each ofthese apparatuses which receive this BPDU changes logical statuses inports which constitute a loop into blocking statuses based upon theprotocol information of the received BPDU, and constructs such a networkhaving a tree structure, so that the loop is logically cut off.

While the redundant protocol described in JP-A-2004-201009 isspecifically used in a ring network, one structural apparatus amongapparatuses which constitutes the ring network is defined as amonitoring apparatus, whereas other structural apparatuses are definedas relay apparatuses. In the monitoring apparatus, a control frame isperiodically transmitted from an one-sided port thereof, and a receptionof this transmitted is monitored at an opposite-sided port thereof.While the control frame is being received at the opposite-sided port,the monitoring apparatus transmits another control frame from thisopposite-sided port, and sets the opposite-sided port to such acondition that a user frame cannot be relayed. As a consequence, theloop structure is logically cut off. If the control frame cannot bereceived at the opposite-sided port, then the monitoring apparatus stopsthe transmission of the control frame from this opposite-sided port, andpermits the relay of the user frame. As a result, the communicationroutes can be made redundant.

Within the above-described conventional techniques, since the spanningtree protocol disclosed in IEEE Std 802.1D 1988 Edition may also bedirected to complex network structures, the tree structure must bedetermined based upon the plurality of information superimposed on BPDUexchanged among the apparatuses, and thus, the protocol operationsbecome complex. As a result, even when a structure of a ring network ismade simple, the below-mentioned problems may occur. That is, while thestructure of such a simple ring network is changed when a failure occursand the failure is recovered within this simple ring network, and alsowhile apparatuses are additionally conducted to the simple ring network,there are some possibilities that several tens of seconds are requireduntil communications are stabilized, during which the communications arestopped. Also, since it is practically difficult to realize the complexprotocol operations by employing hardware, these complex protocoloperations are realized by employing software. As a result, if loadsgiven to a CPU are increased, then there is another problem that anadverse influence may be given to the complex protocol operations.

SUMMARY OF THE INVENTION

Further, as to the redundant protocol described in JP-A-2004-201009within the conventional techniques, the problems occurred in thespanning tree protocol have been solved by specifying the redundantprotocol to the ring network. However, this redundant protocol has thebelow-mentioned problems to be solved. That is, since the control frameis transmitted from only one-sided port (namely, only one direction) ofthe monitoring apparatus provided in the ring network, in such a casethat a failure of one direction happens to occur in a link whichconstitutes the ring network, the monitoring apparatus judges that aring status is under abnormal condition, and thus, permits theopposite-sided port to relay a user frame, so that a ring status of theother direction is brought into a loop status. Furthermore, in anothernetwork structure that a plurality of ring networks are coupled to eachother and coupled apparatuses become redundant, there is such alimitation that an apparatus at the coupling portion must be amonitoring apparatus. As a consequence, there is another problem to besolved that the network structure cannot have a flexible networkstructure. Also, as a problem specific to a ring network, in the casethat the plurality of ring networks are coupled to each other andapparatuses of coupling portions are made redundant, if a failurehappens to occur in a link between the apparatuses made redundant, thenthe following problem occurs. That is, in the coupled ring networks,there are certain possibilities that a large loop structure may beformed.

An object of the present invention has been made to solve theabove-described problems, and therefore, is to provide an apparatuscapable of realizing high-speed switching of communication routes,capable of preventing an occurrence of a loop which is caused by anone-directional failure of a link within a ring network, and capable ofrealizing a simple and flexible structure constructed by coupling aplurality of ring networks, and further, capable of avoiding aconstruction of a loop caused by a failure of a specific link.

To achieve the above-described object, an apparatus according to thepresent invention is featured as follows: That is, as apparatuses whichconstruct a ring network, such a structure made of a monitoringapparatus and other relay apparatuses is employed. The monitoringapparatus monitors only a ring status of such a ring network to whichthe own monitoring apparatus belongs. The monitoring apparatusperiodically transmits health check frames from ring ports thereofrespectively so as to monitor whether or not the transmitted healthcheck frame has been received by the ring port located opposite thereto.Also, in a structure of a multi-ring network having a shared link,auxiliary health check frames are periodically transmitted from two setsof shared apparatuses respectively with respect to a monitoringapparatus of a shared link non-monitored ring network. The monitoringapparatus also monitors the auxiliary health check frames in combinationwith the health check frames in order to avoid an occurrence of a loopwhich bridges a plurality of ring networks. Since the above-describedapparatus is provided, the above-described object may be achieved.

In accordance with the present invention, the monitoring apparatus whichconstructs the ring network judges whether or not relaying of a userframe is permitted by checking whether or not the periodicallytransmitted health check frame is received. As a consequence, thehigh-speed switching of the routes can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B are diagrams for schematically showing a monitoringapparatus employed in a ring network constructing apparatus according toan embodiment of the present invention.

FIG. 2A and FIG. 2B are diagrams for indicating a relay apparatusemployed in the ring network constructing apparatus of the embodiment.

FIG. 3A and FIG. 3B are diagrams for representing a monitoring apparatusin a shared link non-monitored ring network of the ring networkconstructing apparatus of the embodiment.

FIG. 4A and FIG. 4B are diagrams for showing a relay apparatus (which iscommonly operable as shared apparatus) in the shared link non-monitoredring network of the ring network constructing apparatus of theembodiment.

FIG. 5A and FIG. 5B are diagrams for showing a relay apparatus in theshared link non-monitored ring network of the ring network constructingapparatus of the embodiment.

FIG. 6 is a structural diagram of a single ring network with employmentof the ring network constructing apparatus of the embodiment.

FIG. 7 is another structural diagram of the single ring network withemployment of the ring network constructing apparatus of the embodiment.

FIG. 8 is a structural diagram of a multi-ring network (having no sharedlink) with employment of the ring network constructing apparatus of theembodiment.

FIG. 9 is another structural diagram of the multi-ring network (havingno shared link) with employment of the ring network constructingapparatus of the embodiment.

FIG. 10 is a structural diagram of a multi-ring network (having sharedlink) with employment of the ring network constructing apparatus of theembodiment.

FIG. 11 is another structural diagram of the multi-ring network (havingshared link) with employment of the rink network constructing apparatusof the embodiment.

FIG. 12 is a structural diagram of a multi-ring network (having noshared link) with employment of the ring network constructing apparatusof the embodiment.

FIG. 13 is a structural diagram of a multi-ring network (having noshared link) with employment of the ring network constructing apparatusof the embodiment.

FIG. 14 is a structural diagram of a multi-ring network (having noshared link) with employment of the ring network constructing apparatusof the embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to drawings, a detailed description is made of a ringnetwork constructing apparatus according to an embodiment of the presentinvention.

FIG. 1A and FIG. 1B schematically show an apparatus 101 (will bereferred to as “monitoring apparatus” hereinafter) which monitors a ringnetwork employed in a ring network constructing apparatus according toan embodiment of the present invention. The monitoring apparatus 101constitutes a portion of a ring network indicated, for example, in FIG.6, and corresponds to an apparatus 601 which monitors this ring network.The ring network is arranged by the monitoring apparatus 601 and otherrelay apparatuses 602, 603, and 604. Although a detailed description asto FIG. 6 will be made later, in this example, for instance, operationsof the monitoring apparatus 601 in the ring network of FIG. 6 aredescribed with reference to FIG. 1A and FIG. 1B. The monitoringapparatus 101 contains two pieces of ports (will be referred to as “ringports” hereinafter) 102 and 103, which constitute the same ring network.The monitoring apparatus 101 periodically transmits a control frame 111from the ring port 102 so as to monitor whether or not a health checkframe 113 via other apparatuses which constitute the ring network hasbeen received at the ring port 103. The control frame 111 (will bereferred to as “health check frame” hereinafter) monitors such a status(will be referred to as “ring status” hereinafter) as to whether or notthe ring network has performed a communicating operation under normalcondition, and after the health check frame 113 has been received, thishealth check frame 113 is discarded. Similarly, the monitoring apparatus101 periodically transmits a health check frame 112 from the ring port103 so as to monitor whether or not this health check frame 114 has beenreceived at the ring port 102, and after this health check frame 114 hasbeen received, the received health check frame 114 is discarded. Whileany one of the health check frame 113 and the health check frame 114 isbeing received, the monitoring apparatus 101 judges that the ring statusis under normal condition, and brings the ring port 103 into such acondition that a user frame is not transmitted/received in order notgenerate a loop (FIG. 1A). When both the health check frames 113 and 114cannot be received, the monitoring apparatus 101 judges that the ringstatus is under abnormal condition, and transfers the ring port 103 tosuch a condition that the user frame is transmitted/received (FIG. 1B).In this example, the ring port 103 controls whether or not thetransmission/reception of the user frame are permitted in response tothe ring status. Alternatively, the ring port 102 may control whether ornot the transmission/reception of the user frame are permitted inresponse to the ring status. A ring port for controlling whether or notthe transmission/reception of the user frame are permitted may beautomatically determined based upon the numbers of the two ring ports,or may be alternatively designated based upon a configuration by a user.

It should also be noted that in the monitoring apparatus 101, the healthcheck frames 111 and 112 are periodically transmitted from the two ringports 102 and 103, and the reception of the health check frames 111 and112 are monitored at the ring ports which are different from those usedin the frame transmissions. Alternatively, the health check frames 111and 112 may be periodically transmitted only from any one of the ringports 102 and 103, and the received health check frames 111 and 112 maybe monitored only at the other ring port thereof. Also, at this time,while the health check frames 111 and 112 can be received at the otherring port, the monitoring apparatus 101 may judge that the ring statusis under normal condition, and defines this ring status as such a status(which is painted in solid condition in drawings) that a user frame isnot transmitted/received at any one of these two ring ports 102 and 103.Also, another status that the user frame is transmitted/received at theother ring port thereof is indicated in a blank condition. When thehealth check frames 112 and 113 cannot be received at the other ringport, the monitoring apparatus 101 judges that the ring status is underabnormal condition, and changes such a ring port from which thetransmissions/reception of the user frame are not permitted into acondition under which the user frame is transmitted/received.Thereafter, the monitoring apparatus 102 clears MAC address informationand transmits flush request frames from the two ring ports 102 and 103,and when this flush request frame is received at the opposite ring port,the received flush request frame may be discarded.

Further, in the case that in the monitoring apparatus 101, the ringstatus is changed (ring status is changed from normal condition toabnormal condition, or from abnormal condition to normal condition) bypermitting, or not permitting the receptions of the health frames 113and 114, the monitoring apparatus 101 transmits a control frame 131(will be referred to as “flush request frame” hereinafter) from the ringport 102 with respect to other apparatus (will be referred to as “relayapparatus” hereinafter) which constitutes the ring network, and when aflush request frame 132 is received at the ring port 103, the monitoringapparatus 101 discards the received flush request frame 132. Theabove-described control frame 131 instructs to clear MAC addressinformation which has been studded by receiving a user frame withrespect to the relay apparatus. Similarly, in such a case that themonitoring apparatus 101 transmits a flush request frame 133 from thering port 103 and receives a flush request frame 134 at the ring port102, the monitoring apparatus 101 discards the received flush requestframe 134. In the monitoring apparatus 101, when the ring status ischanged, this monitoring apparatus 101 also clears the MAC addressinformation which has been learned by receiving the user frame.Generally speaking, MAC address information which has been learned byreceiving a user frame implies, for example, such MAC addressinformation that port numbers for transferring combinations between MACaddress and VLAN information to a key have been held as a table.

FIG. 2A and FIG. 2B show another relay apparatus 201 which constitutesthe ring network. The relay apparatus 201 corresponds to, for example,relay apparatuses 602, 603, and 604 other than a monitoring apparatus601, which constitute a ring network shown in FIG. 6. The relayapparatus 201 contains two ring ports 202 and 203, which constitute thesame ring network. When a health check frame 211 transmitted from amonitoring apparatus is received at the ring port 202, the relayapparatus 201 relays a health check frame 212 to the ring port 203.Similarly, when a health check frame 213 is received at the ring port203, the relay apparatus 201 relays a health check frame 214 to the ringport 202 provided on the opposite side (FIG. 2A). In the relay apparatus201, only health check frames are relayed, but are not monitored.

Also, for example, in such a case that due to a failure of the ringnetwork, a ring port to which a received health check frame istransferred is brought into a link down condition, the health checkframe cannot be transferred to a succeeding apparatus from a relayapparatus brought into the link down condition. As a result, the healthcheck frame is not transferred up to a monitoring apparatus. In thiscase, the monitoring apparatus judges that the ring status is underabnormal condition. However, if the network failure is recovered and thelink down condition is changed into a link up condition, then a healthcheck frame can be again transferred. As a result, the monitoringapparatus judges that the ring status becomes a normal condition.

When a flush request frame 221 transmitted from a monitoring apparatusis received at the ring port 202 in the relay apparatus 201, the relayapparatus 201 relays a flush request frame 222 to the ring port 203, andat the same time, clears MAC address information which has been learnedby receiving a user frame. Similarly, when a flush request frame 223transmitted from a monitoring apparatus is received at the ring port 203in the relay apparatus 201, the relay apparatus 201 relays a flushrequest frame 224 to the ring port 202 provided on the opposite side,and at the same time, clears the MAC address information which has beenlearned by receiving the user frame (FIG. 2B). It should also be notedthat when the relay apparatus 201 has already cleared the MAC addressinformation by receiving the previous flush request frame 221, the relayapparatus 201 does not clear the MAC address information by receivingthe flush request frame 223.

When either the ring port 202 or the ring port 203 has been brought intothe link down condition, the condition of the relay apparatus 201 isbrought into such a condition that a control frame and a user frame arenot relayed. However, when this link down condition is recovered to alink up condition, the condition of the relay apparatus 201 is changedinto such a condition that the control frame is relayed, and the userframe is not relayed. This reason is given as follows: That is, when thecondition of the relay apparatus 201 is brought into such a conditionthat the relay apparatus 201 relays the user frame just after the linkdown condition is recovered to the link up condition, if the monitoringapparatus has still set to such a condition that one of the ring ports202 and 203 is used to transmit/receive the user frame, then a loop mayoccur. Then, under this condition, when the relay apparatus 201 receivesany one of the flush request frames 221 and 223 transmitted from themonitoring apparatus, the condition of the relay apparatus 201 ischanged into such a condition that the user frame is relayed. Otherwise,even when a flush request frame has not yet been received, after apredetermined time has passed, the condition of the relay apparatus 201is changed into the condition that the user frame is relayed.

Next, a description is made of a network structure in which a pluralityof ring networks have been coupled to each other, which will be referredto as a multi-ring network hereinafter. The multi-ring network has beenarranged in such a manner that, for instance, as represented in FIG. 10,two sets of ring networks 1061 and 1062 have been coupled to each otherby employing shared apparatuses 1002 and 1003. In this case, 2 sets, ormore sets of ring networks to be coupled to each other may bealternatively employed, and 2 pieces, or more pieces of sharedapparatuses may be alternatively employed. In the two ring networks 1061and 1062, monitoring apparatuses 1001 and 1005 are present respectively.While control frames which are transmitted by the monitoring apparatuses1001 and 1005 are transferred only within the respective ring networks1061 and 1062, both the control frames of the monitoring apparatuses1001 and 1005 are transferred between the shared apparatus 1002 and theshared apparatus 1003 (will be referred to as “shared link”hereinafter). In the present embodiment, while the monitoringapparatuses 1001 and 1005 monitor ring status of the respective ringnetworks 1061 and 1062, as to the shared link, only the monitoringapparatus 1001 monitors, whereas the monitoring apparatus 1005 monitorsring status of the ring network other than the shared link within thering network 1062. As a consequence, these ring networks are dividedinto a ring network (will be referred to as “shared link monitored ringnetwork” hereinafter) 1061 which monitors the ring status containingalso the shared link, and another ring network (will be referred to as“shared link non-monitored ring network” hereinafter) 1062 whichmonitors the ring status which do not contain the shared link. At thistime, the shared link monitored ring network 1061 and the shared linknon-monitored ring network 1062 correspond to independent ring networksrespectively. It should also be noted that a detailed description as toFIG. 10 will be discussed later.

Next, a description is made of operations of the respective apparatuseswhich constitute the multi-ring network. Since the shared link monitoredring network 1061 is identical to such a ring network shown in FIG. 6,the monitoring apparatus 1001 performs the same operation as that of themonitoring apparatus 101 described in FIG. 1A and FIG. 1B, whereas theshared apparatuses 1002 and 1003, and the relay apparatus 1004 performthe same operations as those of the relay apparatus 201 explained inFIG. 2A and FIG. 2B. As a result, the shared link monitored ring network1061 is monitored.

Next, a description is made of operations of the respective apparatusesemployed in the shared link non-monitored ring network 1062.

FIG. 3A and FIG. 3B indicate a monitoring apparatus 301 provided in theshared link non-monitored ring network 1062. The monitoring apparatus301 contains two pieces of ring ports 302 and 303, which constitute thesame ring network. The monitoring apparatus 301 periodically transmits acontrol frame 311 (will be referred to as “health check frame”hereinafter) for monitoring a ring status from the ring port 302 via arelay apparatus so as to monitor whether or not a health check frame 312has been received at the ring port 302, and after the health check frame312 has been received, this health check frame 312 is discarded.Similarly, the monitoring apparatus 301 periodically transmits a healthcheck frame 313 from the ring port 303 so as to monitor whether or notthis health check frame 314 has been received at the ring port 302, andafter this health check frame 314 has been received, the received healthcheck frame 314 is discarded. Also, in combination with this operation,the monitoring apparatus 301 monitors both an auxiliary health checkframe (will be referred to as “auxiliary health check frame”hereinafter) 315, and another auxiliary health check frame 316, andafter these auxiliary health check frames 315 and 316 have beenreceived, these received auxiliary health check frames 315 and 316 arediscarded. The auxiliary health check frames 315 and 316 areperiodically transmitted from a shared apparatus 401 which will bediscussed later in FIG. 4A and FIG. 4B with respect to the ring port 302and to the ring port 303 respectively. While any one of the health checkframe 312 and the health check frame 314 is being received, themonitoring apparatus 301 judges that the ring status is under normalcondition, and sets the ring port 302 into such a condition that a userframe is not transmitted/received (FIG. 3A). Although both the healthcheck frames 312 and 314 cannot be received, while both the auxiliaryhealth check frames 315 and 316 are being received, the monitoringapparatus 301 judges that the ring status is under normal condition, andsets the ring port 303 to such a condition 321 that the user frame isnot transmitted/received, since it is conceivable that the apparatusesother than the shared link which should be monitored are under normalconditions. When both the health check frames 312 and 314 cannot bereceived, but also, any one of the auxiliary health check frames 315 and316 cannot be received, the monitoring apparatus 301 judges that thering status is under abnormal condition, and thus, transfers the ringport 303 to such a condition 322 that the user frame istransmitted/received (FIG. 3B). In this case, although the ring port 303controls whether or not the transmission/reception of the user frame arepermitted in response to the ring status, the ring port 302 may controlwhether or not the transmission/reception of the user frame arepermitted in response to the ring status. A ring port for controllingwhether or not the transmission/reception of the user frame arepermitted may be automatically determined based upon the numbers of thetwo ring ports, or may be alternatively designated based upon aconfiguration by a user.

Further, in the case that in the monitoring apparatus 301, the ringstatus is changed (ring status is changed from normal condition toabnormal condition, or from abnormal condition to normal condition) bypermitting, or not permitting the receptions of the health frames 312and 314, the monitoring apparatus 301 transmits a control frame (will bereferred to as “flush request frame” hereinafter) 331 from the ring port302 with respect to other apparatuses which constitute the ring network,and when a flush request frame 332 is received at the ring port 303, themonitoring apparatus 301 discards the received flush request frame 332.The above-described control frame 331 instructs to clear MAC addressinformation which has been learned by receiving a user frame withrespect to other apparatuses. Similarly, in such a case that themonitoring apparatus 301 transmits a flush request frame 333 from thering port 303 and receives a flush request frame 334 at the ring port302, the monitoring apparatus 301 discards the received flush requestframe 334. In the monitoring apparatus 301, this monitoring apparatus301 also clears the MAC address information which has been learned byreceiving the user frame.

FIG. 4A and FIG. 4B show a shared apparatus 401 employed in the sharedlink non-monitored ring network 1062. The shared apparatus 401 has tworing ports 402 and 403, which constitute the same ring network. The ringport 403 corresponds to a port which is used as a shared link withanother ring network. The shared apparatus 401 periodically transmits anauxiliary health check frame 415 from the ring port 402 toward themonitoring apparatus 301 of the ring network, which does not monitor theshared link. Also, since the shared apparatus 401 also constitutes therelay apparatus, when a health check frame 411 transmitted from themonitoring apparatus 301 is received at the ring port 402, the sharedapparatus 401 relays a health check frame 412 to the ring port 403.Also, similarly, when a health check frame 413 is received at the ringport 403, the shared apparatus 401 relays a health check frame 414 tothe ring port 402 provided on the opposite side (FIG. 4A).

It should also be noted that such a fact whether or not a health checkframe transmitted from the monitoring apparatus 301 of the shared linknon-monitored ring network 1062 has been received may be alternativelysuperimposed on the auxiliary health check frame 415 which istransmitted from the shared apparatus 401. Also, while the sharedapparatus 401 does not periodically transmit the auxiliary health checkframe 415 which is transmitted from the shared apparatus 401, the sharedapparatus 401 may alternatively transfer such a health check frametransmitted from the monitoring apparatus 301 to the next apparatus, andalso may fold back this transferred health check frame, and then, mayalternatively transmit the folded health check frame to the monitoringapparatus 301 as the auxiliary health check frame 415.

Also, when the shared apparatus 401 receives a flush request frame 421at the ring port 402, which is transmitted from the monitoring apparatus301, the shared apparatus 401 relays a flush request frame 422 to thering port 403, and at the same time, clears MAC address informationwhich has been learned by receiving a user frame. Similarly, when theshared apparatus 401 receives a flush request frame 423 at the ring port403, the shared apparatus 401 relays a flush request frame 424 to thering port 402 provided on the opposite side, and at the same time,clears the MAC address information. It should be noted that when theshared apparatus 401 has already cleared the MAC address information byreceiving the previous flush request frame 421, the shared apparatus 401does not clear the MAC address information by receiving the flushrequest frame 423.

When either the ring port 402 or the ring port 403 has been brought intothe link down condition, the condition of the shared apparatus 401 isbrought into such a condition that a control frame and a user frame arenot relayed. However, when this link down condition is recovered to alink up condition, the condition of the shared apparatus 401 is changedinto such a condition that the control frame is relayed, and the userframe is not relayed. This reason is given as follows: That is, when thecondition of the shared apparatus 401 is brought into such a conditionthat the shared apparatus 401 relays the user frame just after the linkdown condition is recovered to the link up condition, if the monitoringapparatus has still set to such a condition that one of the ring ports402 and 403 is used to transmit/receive the user frame, then a loop mayoccur. Then, under this condition, when the shared apparatus 401receives any one of the flush request frames 421 and 423 transmittedfrom the monitoring apparatus, the condition of the shared apparatus 401is changed into such a condition that the user frame is relayed.Otherwise, even when a flush request frame has not yet been received,after a predetermined time has passed, the condition of the sharedapparatus 401 is changed into the condition that the user frame isrelayed.

FIG. 5A and FIG. 5B indicate a relay apparatus 501 provided in theshared link non-monitored ring network 1062. The relay apparatus 501contains two ring ports 502 and 503, which constitute the same ringnetwork. When the relay apparatus 501 receives either a health checkframe 511 transmitted from a monitoring apparatus or an auxiliary healthcheck frame 515 transmitted from a shared apparatus, the relay apparatus501 relays either a health check frame 512 or a health check frame 516to the ring port 503. Similarly, when the relay apparatus 501 receivesthe health check frame 513 at the ring port 503, the relay apparatus 501relays a health check frame 514 to the ring port 202 located oppositeside thereof (FIG. 5A).

Also, when the relay apparatus 501 receives a flush request frame 521 atthe ring port 502, which is transmitted from the monitoring apparatus501, the relay apparatus 501 relays a flush request frame 522 to thering port 503, and at the same time, clears MAC address informationwhich has been learned by receiving a user frame. Similarly, when therelay apparatus 501 receives a flush request frame 523 at the ring port503, the relay apparatus 501 relays a flush request frame 524 to thering port 502 provided on the opposite side, and at the same time,clears the MAC address information. It should be noted that when therelay apparatus 501 has already cleared the MAC address information byreceiving the previous flush request frame 521, the relay apparatus 501does not clear the MAC address information by receiving the flushrequest frame 523.

When either the ring port 502 or the ring port 503 has been brought intothe link down condition, the condition of the relay apparatus 501 isbrought into such a condition that a control frame and a user frame arenot relayed. However, when this link down condition is recovered to alink up condition, the condition of the relay apparatus 501 is changedinto such a condition that the control frame is relayed, and the userframe is not relayed. This reason is given as follows: That is, when therelay apparatus 501 is brought into such a condition that the relayapparatus 501 relays the user frame just after the link down conditionis recovered to the link up condition, if the monitoring apparatus hasstill set to such a condition that one of the ring ports 502 and 503 isused to transmit/receive the user frame, then a loop may occur. Then,under this condition, when the relay apparatus 501 receives any one ofthe flush request frames 521 and 523 transmitted from the monitoringapparatus, the condition of the relay apparatus 501 is changed into sucha condition that the user frame is relayed. Otherwise, even when a flushrequest frame has not yet been received, after a predetermined time haspassed, the condition of the relay apparatus 501 is changed into thecondition that the user frame is relayed.

In the relay apparatus 501 and the shared apparatus 401, when a linkcondition of a ring port is changed, a control frame (will be referredto as “link condition change notification frame” hereinafter) may bealternatively transmitted from a ring port under normal condition, whilethis control frame notifies the change in the link condition. Also, themonitoring apparatus 301 may not only monitor the ring status based uponeither the health check frame or the auxiliary health check frame, butalso may monitor the ring status, while including the reception of thelink condition change notification frame which is transmitted fromeither the relay apparatus 501 or the shred apparatus 401 in addition tothese health check frames. It is so assumed that the transmission timeperiods as to the health check frames and the auxiliary health checkframes, and the time durations required for judging that the ring statusare under the abnormal conditions since the health check frames are notyet received may be changed based upon a configuration by a user. Thesetransmission time periods and time durations have been described in themonitoring apparatuses 101 and 301 shown in FIG. 1A, FIG. 1B, FIG. 3A,FIG. 3B; and also in the shared apparatus 401 shown in FIG. 4A, FIG. 4B.

While the monitoring apparatuses 101 and 301 explained in FIG. 1A, FIG.1B, FIG. 3A, and FIG. 3B are equipped with hardware circuits, thesehardware circuits transmit/receive various sorts of control frames anduser frames from ring ports, and also, monitor whether or not either ahealth check frame or an auxiliary health check frame is received withina predetermined time at a ring port. In such a case that either thehealth check frame or the auxiliary health check frame has not yet beenreceived for the predetermined time, the monitoring apparatuses 101 and301 judge that a ring status is brought into an abnormal condition.Also, while the monitoring apparatuses 101 and 301 monitor thereceptions of the health check frame and the auxiliary health checkframe under such a condition that the ring status is under abnormalcondition, the monitoring apparatus 101 and 301 judge that the ringstatus is under normal condition by receiving these health check frames.It should also be noted that the above-described functions may bealternatively realized by employing software.

While the relay apparatuses 201 and 501 shown in FIG. 2A, FIG. 2B, FIG.5A, and FIG. 5B, and also, the shared apparatus 401 shown in FIG. 4A andFIG. 4B are equipped with hardware circuits, these hardware circuitstransmit/receive various sorts of control frames and user frames, andalso, judge that the ring status is changed.

It should also be noted that the above-described functions may bealternatively realized by employing software.

Based upon FIG. 6, a description is made of operations as to respectiveapparatuses employed in a single ring network (will be referred to as“single ring network” hereinafter). In FIG. 6, among apparatuses 601 to604, according to an embodiment of the present invention, whichconstitute the single ring network, a monitoring apparatus is defined as601, and relay apparatuses are defined as 602 to 604. Although thesingle ring network is constituted by employing 4 sets of theapparatuses 601 to 604, there is no limitation as to a total number ofthese apparatuses.

The monitoring apparatus 601 periodically transmits health check frames611 and 615 from two ring ports thereof. These health check frames 611and 615 are relayed in the relay apparatuses 602 to 604, and then, thehealth check frames 611 and 615 are received at ring ports which aredifferent from the above-described ring ports for transmitting thesehealth check frames 611 and 615. In the case that the health check frame611, or 615 is being received at any one of these two ring ports, themonitoring apparatus 601 judges that a ring status is under normalcondition, and sets such a condition that a user frame is nottransmitted/received at a ring port 621 thereof.

FIG. 7 indicates a condition that a failure 751 occurs in the singlering network of FIG. 6. When the failure 751 happens to occur, a healthcheck frame 711 transmitted from a monitoring apparatus 701 cannot berelayed from a relay apparatus 702 to a succeeding apparatus. Similarly,a health check frame 715 cannot be also relayed from a relay apparatus703 to a succeeding apparatus. As a result, the monitoring apparatus 701cannot receive the health check frames 711 and 715 at the two ring portsthereof, so that the monitoring apparatus 701 judges that a ring statusis under abnormal condition, and changes a ring port 721 thereof intosuch a condition that a user frame can be transmitted/received. Also,the monitoring apparatus 701 clears MAC address information which hasbeen learned by receiving the user frame, and transmits flush requestframes 731 and 735 from the two ring ports thereof with respect to otherrelay apparatuses 702 to 704. The relay apparatuses 702 to 704 whichreceive either the flush request frame 731 or the flush request frame735 clear the MAC address information which has been learned byreceiving the user frame.

When the failure is recovered from the condition where the failure hasoccurred in FIG. 7 and then the failure condition is returned to thenormal condition of FIG. 6, the monitoring apparatus 601 receives anyone of the health check frames 611 and 615 which are transmitted by themonitoring apparatus 601 at the ring port. As a result, the monitoringapparatus 601 judges that the ring status becomes the normal condition,and again changes such a ring port where the transmission/reception ofthe user frame have been permitted into another condition where the userframe is not transmitted/received. Also, the monitoring apparatus 601clears the MAC address information which has been learned by receivingthe user frame, and also, transmits a flush request frame from the tworing ports with respect to other relay apparatuses 602 to 604 in asimilar manner when the failure 751 occurs in FIG. 7. The relayapparatuses 602 to 604 which receive this flush request frame clear theMAC address information which has been learned by receiving the userframe.

Also, the ring ports of the relay apparatuses 602 and 603 where afailure of a link has occurred can transmit/receive such a control frameas a health check frame and a flush request frame when the failure isrecovered, and are brought into such a condition that a user framecannot be transmitted/received. When the relay apparatuses 602 and 603receive the flush request frame, the relay apparatuses 602 and 603change the ring ports where the transmission/receptions of the userframes cannot be permitted into such a condition that the user framescan be transmitted/received. Otherwise, even when the flush requestframe is not received, after a predetermined time has passed, the relayapparatuses 602 and 603 changes the present condition into such acondition that the user frames can be transmitted/received.

Referring now to FIG. 8, a description is made of operations as torespective apparatuses provided in such a structure that a plurality ofring networks are coupled to each other, and furthermore, a couplingapparatus has not been made redundant. This structure corresponds tosuch a network structure that there is no link (namely, shared link)which is shared by the plural ring networks. In FIG. 8, although a totalnumber of the coupled ring networks is selected to be 2, there is nolimitation with respect to a total quantity of these ring networks to becoupled to each other.

In FIG. 8, among apparatuses 801 to 803 which constitute a ring network841, a monitoring apparatus is defined as 801, and relay apparatuses aredefined as 802 and 803. Also, among apparatuses 802, 804, and 805, whichconstitute another ring network 842, a monitoring apparatus is definedas 804, and relay apparatuses are defined as 802 and 805. While therelay apparatus 802 constitutes a coupling apparatus of the ringnetworks 841 and 842, the relay apparatus 802 has constructed the ringnetworks 841 and 842 by two different ring ports with respect to each ofthe ring networks 841 and 842. In this coupling apparatus, controlframes which are transmitted/received in the respective ring networks841 and 842 are relayed by being closed in the respective ring networks841 and 842, but are not transferred to the other ring network. Themonitoring apparatus 801 monitors a ring status of the ring network 841,and sets a ring port 831 thereof to such a condition that a user frameis not transmitted and received. The monitoring apparatus 804 monitors aring status of the ring network 842, and sets a ring port 832 thereof tosuch a condition that a user frame is not transmitted and received. As aresult, the ring networks 841 and 842 can be independently operated.

FIG. 9 indicates a structure made in the case that an apparatus forcoupling two ring networks is used as a monitoring apparatus. In FIG. 9,among apparatuses 901 to 903 which constitute a ring network 941, amonitoring apparatus is defined as 901, and relay apparatuses aredefined as 902 and 903. Also, among apparatuses 902, 904, and 905, whichconstitute another ring network 942, a monitoring apparatus is definedas 901, and relay apparatuses are defined as 904 and 905. While thecoupled apparatus becomes the monitoring apparatus 901, this coupledapparatus constitutes the ring networks 941 and 942 at two differentring ports with respect to each of the ring networks 941 and 942, and acontrol frame is not transferred to the other ring network 941, or 942.The monitoring apparatus 901 monitors the ring network 941, and sets aring port 931 thereof to such a condition that a user frame is nottransmitted and received. At the same time, the monitoring apparatus 901monitors the ring network 942, and sets a ring port 932 thereof to sucha condition that a user frame is not transmitted and received. As aresult, the ring networks 941 and 942 can be independently operated.

Referring now to FIG. 10, a description is made of operations as torespective apparatuses provided in such a structure that a plurality ofring networks are coupled to each other, and furthermore, a couplingapparatus has been made redundant. In FIG. 10, although a total numberof the coupled ring networks is selected to be 2, there is no limitationwith respect to a total quantity of these ring networks to be coupled toeach other.

In FIG. 10, among apparatuses 1001 to 1004 which constitute a ringnetwork 1061, a monitoring apparatus is defined as 1001, and relayapparatuses are defined as 1002 to 1004. Also, among apparatuses 1002,1003, 1005, and 1006, which constitute another ring network 1062, amonitoring apparatus is defined as 1005, and relay apparatuses aredefined as 1002, 1003, and 1006. While the relay apparatus 1002 and 1003constitute coupling apparatuses of the ring networks 1061 and 1062, therelay apparatuses 1002 and 1003 have been constructed as havingredundant structures (will be referred to as “shared apparatus”hereinafter). Also, a link between the shared apparatuses 1002 and 1003constitutes such a link (will be referred to as “shared link”hereinafter) which is shared by the ring networks 1061 and 1062. In theshared apparatuses 1062 and 1063, control frames which aretransmitted/received in the respective ring networks 1061 and 1062 arerelayed by being closed in the respective ring networks 1061 and 1062,but are not transferred to the other ring network. Within the two ringnetworks, the ring network 1061 is defined as such a ring network (willbe referred to as “shared link monitored ring network” hereinafter)which monitors a ring status containing also the shared link, whereasthe ring network 1062 is defined as such a ring network (will bereferred to as “shared link non-monitored ring network” hereinafter)which monitors a ring status which does not contain the shared link. Themonitoring apparatus 1001 of the shared link monitored ring network 1061periodically transmits health check frames 1011 and 1015 from two ringports thereof. These health check frames 1011 and 1015 are relayed viathe relay apparatuses 1002 to 1004, and the monitoring apparatus 1002receives the health check frames 1011 and 1015 at different ring portsfrom the ring ports for transmitting these health check frames 1011 and1015. Within the two ring ports, while the health check frame is beingreceived at any one of these ring ports, the monitoring apparatus 1002judges that the ring status is under normal condition, and sets such acondition that a user frame is not transmitted/received at a ring port1051.

The monitoring apparatus 1005 of the shared link non-monitored ringnetwork 1062 periodically transmits health check frames 1021 and 1025from two ring ports thereof. Also, since the shared nodes 1002 and 1003do not monitor the shared link, the shared nodes 1002 and 1003periodically transmit auxiliary health check frames 1031 and 1032 fromsuch ring ports which are located opposite to the ring ports whichbecome the shred links toward the monitoring apparatus 1005. Theauxiliary health check frames 1031 and 1032 are not transmitted to theshared link monitored ring network 1061. These health check frames 1021and 1025, and these auxiliary health check frames 1031 and 1032 arerelayed in the relay apparatuses (including shared apparatus) 1002,1003, and 1006, and then, the monitoring apparatus 1005 receives thesehealth check frames 1021 and 1025, and these auxiliary health checkframes 1031 and 1032 at the two ring ports thereof. Within the two ringports, while the health check frames 1021 and 1025 are being received atany one of these two ring ports, or while the auxiliary health checkframes 1031 and 1032 transmitted from the shared apparatuses 1002 and1003 are being received at the two ring ports, the monitor apparatus1005 judges that the ring status is under normal condition, and setssuch a condition that the user frame is not transmitted and received atthe ring port 1052.

FIG. 11 indicates a condition that a failure 1171 occurs in the sharedlink monitored ring network 1061 of FIG. 10. When the failure 1171happens to occur, a health check frame 1111 transmitted from amonitoring apparatus 1101 cannot be relayed from a relay apparatus 1103to a succeeding apparatus. Similarly, a health check frame 1115 cannotbe also relayed from a relay apparatus 1104 to a succeeding apparatus.As a result, the monitoring apparatus 1101 cannot receive the healthcheck frames 1111 and 1115 at the two ring ports thereof, so that themonitoring apparatus 1101 judges that a ring status is under abnormalcondition, and changes a ring port 1151 thereof into such a conditionthat a user frame can be transmitted/received. Also, the monitoringapparatus 1101 clears MAC address information which has been learned byreceiving the user frame, and transmits flush request frames 1111 and1115 from the two ring ports thereof with respect to other relayapparatuses 1102 to 1104. The relay apparatuses 1102 to 1104 whichreceive the flush request frames 111 and 1115 clear the MAC addressinformation which has been learned by receiving the user frame. At thistime, since the monitoring apparatus 1105 of the shared linknon-monitored ring network 1162 has already received the health checkframes 1111 and 1115 at the two ring ports thereof, this monitoringapparatus 1105 judges that there is no change in the ring status, and iscontinuously operated.

When the failure is recovered from the condition where the failure hasoccurred in FIG. 11 and then the failure condition is returned to thenormal condition of FIG. 10, the monitoring apparatus 1001 receives anyone of the health check frames 1011 and 1015 which are transmitted bythe monitoring apparatus 1001 at the ring port. As a result, themonitoring apparatus 1001 judges that the ring status becomes the normalcondition, and again changes such a ring port where thetransmission/reception of the user frame have been permitted intoanother condition where the user frame is not transmitted/received.Also, the monitoring apparatus 1001 clears the MAC address informationwhich has been learned by receiving the user frame, and also, transmitsa flush request frame from the two ring ports with respect to otherrelay apparatuses 1002 to 1004 in a similar manner when the failure 751occurs in FIG. 7. The relay apparatuses 1002 to 1004 which receive thisflush request frame clear the MAC address information which has beenlearned by receiving the user frame. At this time, since the monitoringapparatus 1005 of the shared link non-monitored ring network 1062 hascontinuously received the health check frames 1124 and 1125, thismonitoring apparatus 1005 is continuously operated.

FIG. 12 represents such a condition that a shared link failure 1271 ofFIG. 10 happens to occur. When the shared link failure 1271 occurs, ahealth check frame 1211 transmitted from a monitoring apparatus 1201 ofthe shared link monitored network 1261 cannot be relayed from a relayapparatus 1202 to a succeeding apparatus. Similarly, a health checkframe 1215 cannot be also relayed from a relay apparatus 1203 to asucceeding apparatus. As a result, the monitoring apparatus 1201 cannotreceive the health check frames 1211 and 1215 at the two ring portsthereof, so that the monitoring apparatus 1201 judges that a ring statusis under abnormal condition, and changes a ring port 1251 thereof intosuch a condition that a user frame can be transmitted/received. Also,the monitoring apparatus 1201 clears MAC address information which hasbeen learned by receiving the user frame, and transmits flush requestframes 1211 and 1215 from the two ring ports thereof with respect toother relay apparatuses 1202 to 1204. The relay apparatuses 1202 to 1204which receive the flush request frames 1211 and 1215 clear the MACaddress information which has been learned by receiving the user frame.At this time, a health check frame 1221 transmitted from a monitoringapparatus 1205 of a shared link non-monitored network 1262 cannot berelayed from the relay apparatus 1202 to a succeeding apparatus.Similarly, a health check frame 1225 cannot be also relayed from therelay apparatus 1203 to a succeeding apparatus. However, auxiliaryhealth check frames 1231 and 1232 transmitted from the sharedapparatuses 1202 and 1203 can be received at the two ring ports of themonitoring apparatus 1205, so that the monitoring apparatus 1205 judgesthat a ring status except for the shred link is under normal condition,and is continuously operated. In accordance with the above-describedoperations, even when the failure happens to occur in the shared link,it is possible to avoid an occurrence of a large loop which bridges theplurality of ring networks.

When the failure is recovered from the condition where the failure hasoccurred in FIG. 12 and then the failure condition is returned to thenormal condition of FIG. 10, the monitoring apparatus 1001 of the sharedlink monitored ring network 1061 receives any one of the health checkframes 1011 and 1015 which are transmitted by the monitoring apparatus1001 at the ring port. As a result, the monitoring apparatus 1001 judgesthat the ring status becomes the normal condition, and again changessuch a ring port where the transmission/reception of the user frame havebeen permitted into another condition that the user frame is nottransmitted/received. Also, the monitoring apparatus 1001 clears the MACaddress information which has been learned by receiving the user frame,and also, transmits a flush request frame from the two ring ports withrespect to other relay apparatuses 1002 to 1004 in a similar manner whenthe failure 1271 occurs in FIG. 12. The relay apparatuses 1002 to 1004which receive this flush request frame clear the MAC address informationwhich has been learned by receiving the user frame.

At this time, the monitoring apparatus 1005 of the shared linknon-monitored ring network 1062 also receives the health check frames1021 and 1025 transmitted from the monitoring apparatus 1005 at the tworing ports thereof. It should also be noted that since the ring statusremains under the normal condition, the operation is merely continued.

FIG. 13 indicates a condition that a failure 1371 occurs in the sharedlink non-monitored ring network 1062 of FIG. 10. When the failure 1371happens to occur, a health check frame 1321 transmitted from amonitoring apparatus 1305 cannot be relayed from a relay apparatus 1306to a succeeding apparatus. Similarly, a health check frame 1325 cannotbe also relayed from a relay apparatus 1303 to a succeeding apparatus.As a result, the monitoring apparatus 1305 cannot receive the healthcheck frames 1321 and 1325 at the two ring ports thereof. Also, anauxiliary health check frame transmitted from the shared apparatus 1303cannot also be relayed from the relay apparatus 1306 to a succeedingapparatus. An auxiliary health check frame 1331 transmitted from theshared apparatus 1302 is received at the ring port of the monitoringapparatus 1305. As a result, since the health check frames 1321 and 1325transmitted from the monitoring apparatus 1305 cannot be received at thetwo ring ports, and further, the auxiliary health check frametransmitted from the shared apparatus 1303 cannot be received at any oneof the two ring ports, the monitoring apparatus 1305 judges that a ringstatus is under abnormal condition, and changes a ring port 1352 thereofinto such a condition that a user frame can be transmitted/received.Also, the monitoring apparatus 1305 clears MAC address information whichhas been learned by receiving the user frame, and transmits a flushrequest frame from the two ring ports thereof with respect to otherrelay apparatuses 1302, 1303, and 1306. The relay apparatuses 1302,1303, and 1306 which receive the flush request frame clear the MACaddress information which has been learned by receiving the user frame.At this time, since the monitoring apparatus 1301 of the shared linkmonitored ring network 1361 has received the health check frame at thetwo ring ports, the monitoring apparatus 1301 judges that there is nochange in the ring status, and the operation thereof is continued.

When the failure 1371 is recovered from the condition where the failure1371 has occurred in FIG. 13 and then the failure condition is returnedto the normal condition of FIG. 10, the monitoring apparatus 1005receives any one of the health check frames 1021 and 1025 which aretransmitted by the monitoring apparatus 1005 at the ring port.Otherwise, the monitoring apparatus 1005 receives the auxiliary healthcheck frames 1031 and 1033 which are transmitted from the sharedapparatuses 1002 and 1002 at the two ring ports respectively. As aresult, the monitoring apparatus 1005 judges that the ring statusbecomes the normal condition, and again changes such a ring port wherethe transmission/reception of the user frame have been permitted intoanother condition that the user frame is not transmitted/received. Also,the monitoring apparatus 1005 clears the MAC address information whichhas been learned by receiving the user frame, and also, transmits aflush request frame from the two ring ports with respect to other relayapparatuses 1002, 1003, and 1006 in a similar manner when the failure1371 occurs in FIG. 13. The relay apparatuses 1002, 1003 and 1006 whichreceive this flush request frame clear the MAC address information whichhas been learned by receiving the user frame. At this time, since themonitoring apparatus 1005 of the shared link monitored ring network 1061continuously receives the health check frames at the two ring portsthereof, the monitoring apparatus 105 continues the operation thereof.

FIG. 14 shows a structure made in the case that a shared apparatus isused as a monitoring apparatus. Among apparatuses 1401 to 1404 of anembodiment of the present invention, which constitute a shared linkmonitored ring network 1461, a monitoring apparatus is defined as 1401,and relay apparatuses are defined as 1402 to 1404. Also, amongapparatuses 1401, and 1404 to 1406 of the embodiment of the presentinvention, which constitute a shared link non-monitored ring network1462, a monitoring apparatus is defined as 1401, and relay apparatusesare defined as 1404 to 1406. While the apparatus 1401 corresponding tothe monitoring apparatus of each of the ring networks 1461 and 1462constitutes one shared apparatus, this monitoring apparatus 1401independently monitors the respective ring networks 1461 and 1462. Themonitoring apparatus 1401 monitors a ring status as the monitoringapparatus for the shred link monitored network 1461, and sets a ringport 1451 to such a condition that a user frame is not transmitted andreceived. At the same time, the monitoring apparatus 1401 monitors aring status as the monitoring apparatus for the shared linknon-monitored network 1462, and sets a ring port 1452 to such acondition that a user frame is not transmitted and received. At thistime, the ring ports 1451 and 1452 cannot be constituted as ring portson the shared link side, but must be constituted as ring ports which arelocated opposite to the ring ports corresponding to the shared link.Also, since the monitoring apparatus 1401 of the shared linknon-monitored ring network 1462 is commonly employed as the sharedapparatus, the monitoring apparatus 1401 does not transmit such anauxiliary health check frame so as not to monitor the shared link, butmonitors another auxiliary health check frame 1431 transmitted fromanother shared apparatus 1404 in combination with the health checkframes 1421 and 1425 which are transmitted by the own shared apparatus1401. As a result, similar to FIG. 10, the ring networks 1461 and 1462can be independently operated.

In accordance with the monitoring apparatuses in the presentembodiments, the shared apparatuses, and the relay apparatuses of thepresent invention, the below-mentioned effects can be achieved.

(1) In a monitoring apparatus which constitutes a ring network, themonitoring apparatus judges whether or not a relay of a user frame ispermitted by determining whether or not a health check frame is receivedwhich is periodically transmitted. As a result, routes can be switchedin a high speed.

(2) As previously described, since the monitoring apparatus may merelymonitor a reception of a control frame, the system may be easilyrealized in a hardware fashion. As a result, while there is no unwantedprotocol operation, loads given to a CPU can be reduced, so that astable protocol operation can be realized.

(3) Health check frames are transmitted from two ring ports of amonitoring apparatus which constitutes a ring network, and these healthcheck frames are monitored at the two ring ports (namely, health checkframes are monitored in bidirectional manner). As a result, even when afailure happens to occur along one direction in a portion of the linkwhich constitutes the ring network, it is possible to prevent anoccurrence of a loop status in connection with the failure.

(4) In the case that a multi-ring network is constructed, the followingconditions may be merely set: any one of a monitoring apparatus, ashared apparatus, and a relay apparatus may be merely set with respectto this apparatus, and which port may be merely set to a ring port. As aresult, these monitoring apparatus, shared apparatus, and relayapparatus may be simply coupled to the multi-ring network. Also, sinceoperations in the respective rings are independently carried out, a ringnetwork may be readily added to be coupled to the existing ring networkwithout giving an adverse influence to the existing ring network.

(5) In such a case that a multi-ring network is constructed, astructural position of a monitoring apparatus may be freely determinedirrespective of such a fact whether or not an apparatus to be coupledtakes a redundant structure, so that apparatuses may be arranged in aflexible manner. Also, the apparatus to be coupled is not necessarily amonitoring apparatus.

(6) In the case where a multi-ring network is constructed and anapparatus to be coupled takes a redundant structure, an auxiliary healthcheck frame is transmitted from this apparatus having the redundantstructure, and a monitoring apparatus monitors a health check frame incombination with this auxiliary health check frame. As a result, evenwhen a failure happens to occur in a link between apparatuses which aremade redundant, it is possible to avoid that a loop is constructed.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A first communication apparatus to be coupled to a secondcommunication apparatus and a third communication apparatus via a ringnetwork, comprising: a first port to coupled to the ring network; and asecond port to coupled to the ring network; wherein the first port andthe second port is set to receive a control frame from the secondcommunication apparatus and the third communication apparatus, andeither the first port or the second port is set to be disabled to relaya user frame.
 2. The first communication apparatus according to claim 1,when a ring failure is detected on the basis of condition whether thecontrol frame is received or not, the first port and the second port isset to be able to relay a user frame.
 3. The first communicationapparatus according to claim 1, when a ring status is detected as afailure of the ring network, the first port and the second port is setto be able to relay a user frame, when a ring status is detected as anormal status, the first port is permitted to relay a user frame and thesecond port is not permitted to relay a user frame.
 4. The firstcommunication apparatus according to claim 1, wherein the control frameincludes information which indicates ring status of the ring network. 5.The first communication apparatus according to claim 1, furthercomprising learning unit which learns MAC (Media Access Control) addressinformation from the user frame; and when said ring status is changed,clears the MAC address information learned by receiving the user frame.6. The first communication apparatus according to claim 5, wherein thelearning unit further transmits flush request frames from the first andsecond ring ports, while the flush request frames instruct that the MACaddress information is cleared with respect to the second communicationapparatus and the third communication apparatus.
 7. A first node in aring network including a second node and, a third node, comprising: afirst port which is set to receive a control frame for managing ringstatus of a ring network which is constituted by the first node, thesecond node and the third node and which is permitted to relay a userframe; a second port which is set to receive a control frame formanaging ring status of a ring network which is constituted by the firstnode, which is prohibited from relaying a user frame when a normalcondition of a ring status of the ring network, and which is permittedto relay a user frame when a failure of the ring network is detected;and a learning means for learning a MAC (Media Access Control) addresson a basis of the relayed user frame.
 8. The first node according toclaim 7, wherein, when a failure of the ring network is detected thelearning means further clears the MAC address information learned byreceiving the user frame, and transmits flush request frames from thefirst and second ring ports, while the flush request frames instructthat the MAC address information is cleared with respect to the secondnode and the third communication apparatus.
 9. The first node accordingto claim 7, wherein, when a failure of the ring network is detected, thelearning means further transmits flush request frames from the first andsecond ring ports, while the flush request frames instruct that the MACaddress information is cleared with respect to the second node and thethird node.
 10. A first node in a ring network including a second nodeand a third node, comprising: a first port which is set to receive acontrol frame for managing ring status of a ring network which isconstituted by the first node, the second node and the third node andwhich is permitted to relay a user frame; a second port which is set toreceive a control frame for managing ring status of a ring network whichis constituted by the first node, which is prohibited from relaying auser frame when a normal condition of a ring status of the ring network,and which is permitted to relay a user frame when a failure of the ringnetwork is detected as a result of managing ring status.
 11. The firstnode according to claim 10, wherein at least one of the first port andthe second port is to send a flush request to the second node and thethird node for clearing MAC information which has been learned from theuser frame.
 12. A ring network system comprising: a first node; a secondnode; a third node; wherein the first node includes a first port tocoupled to the ring network and a second port to coupled to the ringnetwork both of which are set to receive a control frame from the secondcommunication apparatus and the third communication apparatus, andeither of which is set to be disabled to relay a user frame, the firstnode learns MAC (Media Access Control) address information from the userframe, when the first node determines a failure of a ring network hasoccurred, both of the first port and the second port is set to bepermitted to relay a user frame, and the first node clears the MACaddress information learned by receiving the user frame.
 13. The ringnetwork system according to claim 12, wherein the second node and thethird node learn MAC (Media Access Control) address information from theuser frame, wherein the first node transmits a flush request to thesecond node and the third node via the first port or second port, andthe second node and third node clears MAC address information accordingto the flush request.